1. Field of the Invention
This invention relates to linear materials for fasteners, which are endowed with pearly luster and a method for the production thereof.
2. Description of the Prior Art
Polyester monofilaments, particularly polyethylene terephthalate monofilaments, have been heretofore used for fasteners because they possess many excellent properties. The main parts of a fastener are a pair of tapes and fastening elements attached to the tapes (components for joining the tapes). In the case of a slide fastener, for example, the elements are made of a monofilament in a coiled or zigzagged shape. In some of the conventional slide fasteners, tapes and elements are so prepared that they assume a practically equal color after they are separately dyed prior to assembling. In others, tapes and elements are made of monofilaments so colored with pigments that they assume a varied combination of colors.
In the case of a slide fastener produced in one and the same color, notwithstanding the tapes and the elements thereof have undergone the same dyeing treatment, the color consequently assumed by the tapes is variable with the material of threads forming the tapes and the texture formed by weaving. For the purpose of dyeing the elements in a color matching the color of the tapes, therefore, it is necessary that the dyeability or dye-affinity of the elements be controlled. The control of the dyeability of the elements has been generally implemented by adjusting the draw ratio of the monofilament thereby varying the degree of orientation of the monofilament or by adjusting the temperature of the final heat treatment thereby varying the degree of crystallization or crystallinity.
These methods, however, are at a disadvantage in suffering the elements to incur notable dimensional variation during the forming thereof because the changes in conditions of drawing or temperatures of heat treatment caused on the monofilament notably vary the physical properties, particularly the degree of shrinkage, of the monofilament. For the purpose of imparting highly desirable quality to the produced elements, therefore, the degree of shrinkage of the monofilament and consequently the conditions of drawing or the temperatures of heat treatment to be employed are restricted invariably at the sacrifice of the dyeability and consequently the equality of color.
Incidentally, for the purpose of enabling the fasteners such as slide fasteners and hook-and-loop fasteners, particularly their elements, to present an appearance of high quality, the practice of imparting pearly luster thereto has been in vogue. As means to effect this impartation of pearly luster, a method which resides in adding a pigment capable of conferring a pearly color tone (hereinafter referred to as "pearlescent pigment") is popular. In the case of a linear material for a fastener, especially a monofilament as a raw material for fastener elements, when the pigment is added thereto in the proportion of not less than about 3%, the elements in an undyed state indeed acquire a pearly luster. When these elements are dyed, however, they are not fully colored by the dyeing because the added pearlescent pigment has degraded the dyeability thereof and the color of the monofilament itself has already been turned to opaque white by the pigment. Thus, the elements are dyed very poorly as compared with the tapes of the fastener and the pearly luster imparted thereto is likewise inferior. As a result, the tapes and the elements of a fastener cannot be dyed in matched colors and the produced fastener is deficient in commercial value.
As another means to effect the impartation of pearly luster, a method which consists in mixing a coloring pigment with the pearlescent pigment has been known. According to this method, though the monofilaments are obtained indeed as colored in pearly tones, the production of fasteners in such a huge number of colors as 200 to 300 is difficult to achieve from the practical point of view and is unduly expensive.